| Merkett Premium Cleaning & Care Ltd | Administrative & support | 2025 |
| Mersey Medical Nursing Agency Ltd | Health & social care | 2017 |
| Messing Eton Ltd | Hospitality | 2026 |
| MET Transport Limited | Transport & logistics | 2021 |
| Metal Earth Limited | Wholesale & retail | 2021 |
| Metal Strategies Limited | Wholesale & retail | 2023 |
| Metaverse Entertainment Europe Ltd | Hospitality | 2022 |
| Metaverse Gifts Limited | Wholesale & retail | 2022 |
| MG Xpress Ltd | Manufacturing | 2014 |
| MGD Education Limited | Education | 2023 |
| MGJ Property Ltd | Real estate | 2024 |
| MGN Events Limited | Administrative & support | 2011 |
| MGN Structures Limited | Other services | 2024 |
| MHL Group Holdings Limited | Financial services | 2021 |
| MHL Newco 2 Limited | Financial services | 2022 |
| MHR Cleaning and Maintenance Services Limited | Other services | 2020 |
| MHR Protection Limited | Administrative & support | 2015 |
| MHS Accountants Limited | Professional & scientific | 2026 |
| MI Gente Coffee Limited | Manufacturing | 2025 |
| Miasco Ltd | Transport & logistics | 2022 |
| Michael Chell Limited | Wholesale & retail | 2003 |
| Michael Thomas (Medicolegal) Limited | Health & social care | 2018 |
| Michelle Yodts Chiropody & Podiatry Ltd | Health & social care | 2012 |
| Microhealth Ltd | Wholesale & retail | 2023 |
| Micronel UK Limited | Administrative & support | 2008 |
| Microscope Service & Sales Limited | Manufacturing | 2000 |
| Middlesex Sound & Lighting Limited | Manufacturing | 1983 |
| Midgard Holding Ltd | Real estate | 2022 |
| Midwife Nima Ltd | Health & social care | 2023 |
| Mike Cutter Limited | Professional & scientific | 2025 |
| Mike Fiander Carpentry Ltd | Manufacturing | 2025 |
| Mike Walker Consulting Ltd | Professional & scientific | 2020 |
| Mikkom Limited | Information & communication | 1999 |
| Miles Away Living Ltd | Hospitality | 2026 |
| Miles Offshore Services Limited | Mining & quarrying | 2007 |
| Militare Group Ltd | Professional & scientific | 2026 |
| Milkmind Limited | Manufacturing | 2025 |
| Millace Holdings Limited | Financial services | 2017 |
| Millace Properties Limited | Real estate | 2017 |
| Millars Kitchen Ltd | Hospitality | 2018 |
| Millers Walk (Weald) Management Company Limited | Households | 2023 |
| Millside Place Residents Management Company Limited | Households | 2021 |
| Milltree Accounting Ltd | Professional & scientific | 2019 |
| Mimi & Berry Ltd | Wholesale & retail | 2018 |
| Mimi Bland Ltd | Professional & scientific | 2020 |
| Mimi Pisano Limited | Manufacturing | 2005 |
| Mimisbrazilian Ltd | Health & social care | 2026 |
| Mind Sciences Academy Limited | Education | 2025 |
| Mindeology Ltd | Health & social care | 2022 |
| Mini Town Ltd | Arts & recreation | 2021 |
| Minster Pathways (Colchester) Ltd | Extraterritorial | 1989 |
| Mintcolist Ltd | Administrative & support | 2025 |
| Misbourne House Management Limited | Households | 1997 |
| Missendon Limited | Real estate | 1996 |
| MJF Project Management Ltd | Construction | 2017 |
| MJF Ventures Ltd | Financial services | 2020 |
| MJK Decorating Ltd | Construction | 2025 |
| MJL Property Services Ltd | Construction | 2013 |
| MJM Floors Ltd | Construction | 2013 |
| MK & Saiji Company Limited | Wholesale & retail | 2021 |
| MK Capital Holding Ltd | Professional & scientific | 2025 |
| MK Partners Holdings Limited | Professional & scientific | 2025 |
| MKL CX Ltd | Professional & scientific | 2021 |
| MKM Luxe Suisse Limited | Wholesale & retail | 2016 |
| MKO (Windsor) Limited | Administrative & support | 2005 |
| MKR Inspire Ltd | Professional & scientific | 2022 |
| Mmfn Services Ltd | Construction | 2017 |
| MMI Construction Limited | Construction | 2010 |
| MMI Group Holdings Ltd | Construction | 2023 |
| Mmvsense Limited | Information & communication | 2009 |
| MNA Finance Limited | Financial services | 2016 |
| MNA Sourcing Ltd | Wholesale & retail | 2012 |
| MO Farah | Arts & recreation | 2007 |
| Mobilink Trading Ltd | Wholesale & retail | 2016 |
| Mobilitec Limited | Information & communication | 2011 |
| Mobility Room Ltd | Arts & recreation | 2018 |
| Moda Digital Solutions Limited | Professional & scientific | 2024 |
| Modern Landscapes Maintenance Ltd | Other services | 2023 |
| Modus Care (Plymouth) Limited | Health & social care | 1994 |
| Modus Care Limited | Health & social care | 2000 |
| Molesworth Arms Company Limited | Hospitality | 2015 |
| Mollier Limited | Real estate | 2024 |
| Moment in Time Ltd | Hospitality | 2024 |
| Momentum Insights Limited | Professional & scientific | 2025 |
| Momiji | Health & social care | 2003 |
| Monarch Business Enterprise Limited | Wholesale & retail | 2017 |
| Monitor Limited | Information & communication | 2009 |
| Montadale Park Residents Company Limited | Households | 1994 |
| Montague Rental Properties Ltd | Real estate | 2017 |
| Montana Consultancy Ltd | Professional & scientific | 2009 |
| Montrose Court (London Road) Management Company Limited | Households | 2014 |
| Montrose Kingsland Estate Limited | Real estate | 2015 |
| Moon (Clothing) Limited | Manufacturing | 2016 |
| Moon (Investments UK) Ltd | Real estate | 2016 |
| Moon Wholesale Ltd | Wholesale & retail | 2012 |
| Moon's Information Technology Limited | Information & communication | 2004 |
| Moorland Heritage Limited | Construction | 2022 |
| Moors Medics Limited | Health & social care | 2014 |
| More Than a Backpack Ltd | Information & communication | 2021 |
| Morel Eyewear ( U.K.) Limited | Wholesale & retail | 2018 |